Verizon is among dozens of the nation’s leading employers — including Amazon, Goldman Sachs, Target and Facebook itself — that placed recruitment ads limited to particular age groups, an investigation by ProPublica and The New York Times has found.

The ability of advertisers to deliver their message to the precise audience most likely to respond is the cornerstone of Facebook’s business model. But using the system to expose job opportunities only to certain age groups has raised concerns about fairness to older workers.

​Several experts questioned whether the practice is in keeping with the federal Age Discrimination in Employment Act of 1967, which prohibits bias against people 40 or older in hiring or employment. Many jurisdictions make it unlawful to “aid” or “abet” age discrimination, a provision that could apply to companies like Facebook that distribute job ads.